But the greater deference his employees paid to the intelligence and wit of their new CEO had a downside, too. Mr. Bass says he couldn't be as frank and critical as he had been when chief operating officer. He feared employees would overreact. "When you're CEO, what you're saying is so amplified you have to be very careful," says Mr. Bass.

His experience shows why the gap between No. 1 and No. 2 in a company is often bigger than many realize. CEOs not only perform different tasks from their second-in-commands -- who typically focus on running operations -- but they have to act differently, too. That means the two roles often demand very different personality traits, say people who have been there.

CEOs talk about getting acclimated to the limelight. Longtime chief operating officers say they are used to working behind the scenes and submerging their egos. Their jobs focus them inward on the company's problems, while CEOs spend much of their time convincing outsiders of the company's strengths.

The very talents that make a great chief operating officer -- like finicky attention to detail -- can get in the way when you are in the top seat. CEOs are supposed to strategize, not micromanage.

When board members go searching for a new No. 2, they must decide whether they want someone who is a solid operations specialist or someone they're grooming to be No. 1. If they want a chief operating officer with the ambition and drive of a CEO, they may find a No. 2 who doesn't have the patience to play second fiddle for long, says John Thompson, a vice chairman at executive-search firm Heidrick & Struggles International, who has handled CEO and chief operating officer searches for companies such as Google and Dell. Or they may find their newest executive reluctant to take the next step.

The Chief Operating Officer Business Forum, a network of California operating chiefs of mid- to large-size companies this month discussed with its members the pros and cons of moving up to CEO. About half of the 40 members present at the discussions said they didn't "want the headache" of the top post, says William E. Shepard, founder of the forum.

"There was a lot of talk about the persona," says Mr. Shepard, who has held both roles. As CEO, "you have to have the fearless willingness to put yourself out there. Some people said, "I don't want to do that. It's not who I am.'"

Murray Martin felt the role of CEO was a natural fit for him. He became CEO of mail-processing equipment maker Pitney Bowes in May, after three years as operating chief, although he worked under the same CEO for 10 years in a variety of jobs.

But Mr. Martin says he can understand how executives with more retiring personalities wouldn't like the transition. One of the biggest differences between the two jobs, he explains, is how much time is spent on external affairs. When he was operating chief, Mr. Martin says he did speaking engagements only when he wanted to. Now, he is talking to the public at least once or twice a week, including investor meetings, lectures on the company's diversity initiatives, and speeches to groups such as the Conference Board on how to expand through acquisition.

"The CEO has to be the real face of the company in all areas," says Mr. Martin.

CEOs and operating officers can, of course, have traits in common. Both jobs call for a high degree of leadership skills, intelligence and business smarts. A company might work to make the two jobs more in sync, to smooth the way for a move up the ladder.

In that case, the deputy executive would be given the kind of strategy-setting responsibilities more commonly associated with CEOs and pushed into external roles, talking to the company's board and investors. That was the case with Autodesk's Mr. Bass, who became operating chief in 2004.

But even Mr. Bass says he didn't covet the CEO role before he was tapped. Some aspects of the top job -- such as lobbying politicians -- don't really suit his personality, he says. It is important for the CEO to weigh in on issues such as patent or immigration reform, but "it feels like a huge waste of time," confesses Mr. Bass.

He tends to be more interested in products than public relations, yet he must be a cheerleader for the company in front of analysts, shareholders and the community. It's vital that a CEO consistently project a positive attitude to help keep up morale. In the No. 2 role, it was OK "to be more curmudgeonly," he says.

In the end, he'd advise chief operating officers considering the leap to CEO to take a hard look at their personal tastes. "Figure out the things you like best about your job," he says, "then figure out what you have to do as CEO and whether you can put up with it."

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